1. Field of the Invention
The present invention relates to a heat treatment technique employed in a process for manufacturing semiconductor devices, and more particularly to an apparatus and method for manufacturing semiconductor devices, which are planned to reduce the degree of non-uniformity of heating when an impurity diffusion region, for example, is formed, and also relates to a semiconductor device manufactured by the apparatus and method.
2. Description of the Related Art
To appropriately form functional regions, such as source/drain regions, channel regions, etc., it is necessary to appropriately heat (anneal) semiconductor substrates (wafers) when or after ions are injected into them. For example, wafers are heated to approx. 1000° C. for approx. thirty minutes. However, heating at such a high temperature for such a long period causes diffusion of an impurity while the impurity is being activated. This makes it difficult to appropriately form the source/drain regions, channel regions, etc.
To activate an impurity while suppressing the diffusion of the impurity, so-called rapid thermal anneal (RTA), which requires only a short heat treatment period, has been introduced. In RTA, a tungsten filament halogen lamp, for example, is used as a heating device, and wafers are heated to approx. 1000° C. for approx. ten seconds. Japanese Patent No. 2515883, for example, discloses this technique. However, even RTA has come to cause diffusion of impurities as microfabrication of semiconductor devices has been developed. Thus, it has become difficult to acquire a desired impurity profile.
In light of the above, annealing that uses a flash lamp has been highlighted as one for enhancing the rate of activation of an impurity in a shorter period. This flash lamp contains, for example, a gas such as xenon. This technique is disclosed in, for example, Japanese Patent Application KOKAI No. 2001-319887. The conditions of annealing using the flash lamp are, for example, a current-carrying period of approx. 10 ms or less, and an emission energy density of approx. 100 J/cm2 or less.
However, if the heating period is shortened in a heating method using a lamp, it becomes difficult to maintain the in-plane uniformity of the heated state as the surface area of an object to be processed such as a wafer increases. For example, assume that after light from a flash lamp is emitted onto a wafer into which ions are injected, the sheet resistance of an impurity diffusion layer formed by the light irradiation is measured. There is a tendency in which the sheet resistance is lowest just below the lamp, and increases away from the lamp. In other words, it is possible that variations in the in-plane sheet resistance of the wafer may fall outside an allowable range. This is because of the following reason. Just below the lamp, the light emitted from the lamp enters the surface of the wafer at substantially 0°. On the other hand, as the distance from the lamp increases, the angle of incidence of the light entering the surface of the wafer increases. Accordingly, the energy density of the light is lower in the area away from the lamp than in the area just below the lamp. As a result, the light intensity (heat amount) varies over the wafer surface, thereby causing uneven heating. This may cause variations of electric characteristics in the surface of the wafer obtained after heating, and hence degrade the quality of the wafer.
There is a technique for, for example, rotating a wafer during light irradiation (heating) in order to suppress variations of light intensity at the wafer surface. In general, however, since the rotational speed of a wafer is relatively high compared with the period of light irradiation, it is difficult to substantially uniformly irradiate the surface of the wafer with light while the wafer is rotated through 360 degrees. Therefore, this method cannot be applied to the aforementioned RTA or annealing using a flash lamp, in which the light irradiation period is extremely short.
Furthermore, there is a technique for, for example, arranging an opaque quartz plate between a lamp and wafer. In this technique, the opaque quartz plate, which is directly heated by the lamp, substantially uniformly heats the surface of the wafer (i.e., the wafer is indirectly heated by the lamp). In general, however, it takes a relatively long time for the opaque quartz plate to radiate sufficient heat. Accordingly, this method is also inapplicable to, for example, RTA in which the light irradiation period is extremely short.
In addition, there is a technique for interposing between a lamp and wafer a transparent quartz plate that is provided with light shielding members (filters) at selected portions. In this method, the plate has light shielding and passing portions appropriately arranged to suppress variations in light intensity at the surface of a wafer. However, in this technique, the maximum value of the light intensity is reduced by the existence of the light shielding portions, and hence the efficiency of heating degrades. This may lead to an increase in the time required for heating the wafer. Therefore, this method is also inapplicable to, for example, RTA in which the light irradiation period is extremely short.